1. Field of the Invention
The invention relates to a stirred ball mill having a separation member for separating very small grinding media having a diameter D≦0.1 mm from the product.
Stirred ball mills are used, for example, for comminuting and dispersing solids in a liquid phase or for digesting microorganisms.
2. Description of the Prior Art
Stirred ball mills known on the market and intended for continuous operation have a cylindrical grinding chamber with a horizontal or vertical axis and have various types of separation members for separating the grinding media from the product in the product outlet region.
The housing of the grinding chamber of a stirred ball mill consists of a longitudinal wall and two end walls arranged at the two ends of the longitudinal wall. It has a material inlet which serves for feeding the material to be ground and a material outlet which serves for removing the material. Moreover, a stirrer rotatable about the chamber axis is arranged in the grinding chamber for transporting the grinding media present in the mill radially relative to the stirrer shaft and thus comminuting or dispersing, by means of impact and shear forces, the material passed continuously through the grinding chamber.
During the continuous flow through a stirred ball mill, an entraining force which acts on the grinding media occurs independently of the flow velocity and of the viscosity of the suspension of material to be ground. Consequently, the grinding media are entrained with the material to be ground from the entrance into the grinding zone up to the separation membrane in the region of the material outlet. This may lead to compression of the grinding media before the separation member, which compression is associated with increased wear and increased risk of blockage. In the case of very small grinding media having a diameter D≦0.1 mm, the entrainment effect of the product increases several-fold and is critical in the case of very fine milling and dispersing, particularly in the case of high throughput rates.
Stirred ball mills are known which, for solving this problem, have various constructions such that the entrained grinding media are permitted to circulate in the mill with recycling into the grinding zone. However, the known solutions are very often inadequate for grinding media of D≦0.1 mm.
In further known separation systems, such as, for example, with the use of so-called split-sieve cartridges, the production with small gap widths of ≦0.05 mm proves to be very difficult, which gap widths are moreover scarcely usable in practice. In addition, the probability that the sieve will be blocked is very high in these cases if grinding media having D≦0.1 mm have to be used, which is necessary in nanotechnology and hence also in the production of nanoparticles.
A further problem arises through the high pressure drop owing to the accumulation of grinding media on the sieve. In the so-called friction-gap version, extremely high precision is required in production in order to ensure the small friction gaps of ≦0.05 mm in the entire course of production.
The Offenlegungschrift DE 44 12 408 A1 discloses partly keeping the grinding media away from the outlet member in a horizontal mill by means of a preclassification disk and a rotating cage, but grinding media can be entrained through the bores close to the axis in the separation means through which the product has to flow, and said grinding media can thus reach the outlet member. Since the entrainment effect of the material to be ground increases with decreasing diameter of the grinding media, the use of a corresponding separation apparatus which effectively and reliably separates the grinding media from the product is therefore indispensable for such applications.
A blockage of the outlet, possibly caused by the narrow tolerances at the material outlet, can lead to a marked pressure increase and suppress the actual grinding and dispersing process. On failure of the separation function, grinding media moreover emerge from the mill and the grinding and dispersing process deteriorates to an increasing extent. Moreover, in this case, the ground material is contaminated with grinding media.
Further proposals for avoiding the blockage of the separation means are described mainly for vertical stirred ball mills. Such an invention is described in Patent EP 0 771 591 A1. This is a vertical stirred ball mill in which the material to be milled flows upwards through a grinding container loaded with grinding media and a separator resting on the stirrer member before the outflow. Since the separator has no sieve, filter or other system for retaining grinding media, this invention cannot be used in the case of a mill having a horizontal axis since, on filling with grinding media or on emptying the ground material after the end of the milling process, emergence of grinding media at the outlet is to be expected, which contaminates the ground material with grinding media. Furthermore, the centrifugal acceleration of the grinding media is dependent on the velocity of the stirrer disks and hence also on the property of the product which, for example in the case of products sensitive to shearing and hence low speeds, may lead to an excessively low centrifugal acceleration of the grinding media or to the emergence of the grinding media. Vertical stirred ball mills also have the disadvantage of inhomogeneous distribution of grinding media in the grinding chamber, which leads to a poor grinding or dispersing performance.